The present invention generally relates to superconducting circuits and more particularly to a superconducting magnetic sensor that uses a superconducting quantum interference device (SQUID).
Superconducting quantum interference devices (SQUIDs) are used for super-high sensitivity magnetometers suitable for detecting feeble magnetic fields of biological bodies such as brain or heart. In such a medical application of SQUIDs, it is desired to construct the magnetometer to have a large number of channels such that the detection of the magnetic field is achieved simultaneously in such a plurality of channels.
Conventionally, analog SQUIDs that produce an analog output voltage have been used for constructing such super-high sensitivity magnetometers. On the other hand, a magnetometer that uses a digital SQUID has been proposed recently (Fujimaki, N. et al, "A single-chip SQUID magnetometer," IEEE TRANSACTIONS ON ELECTRON DEVICES vol. 35, no. 12, Dec. 1988, pp. 2412-2417), wherein the SQUID magnetometer of the reference is driven by an A.C. bias current and produces output voltage pulses in synchronization with the A.C. bias current as a result of detection of the magnetic field. It will be noted that such a digital output of the magnetometer is suitable for processing in digital circuits.
The foregoing single-chip SQUID magnetometer includes a SQUID sensor and a superconducting detection loop coupled magnetically thereto, and the SQUID sensor produces positive output voltage pulses or negative output voltage pulses depending upon the direction of an unknown magnetic field that interlinks with the superconducting detection loop. The SQUID sensor is biased by an A.C. bias current as mentioned previously and produces the foregoing output voltage pulses in synchronization to such a bias current. Further, the SQUID magnetometer includes a superconducting accumulation loop provided commonly on a chip on which the SQUID sensor is provided, wherein the superconducting accumulation loop is configured to store a flux quantum via a write gate in response to each output voltage pulse produced by the SQUID sensor. The superconducting accumulation loop further produces an analog feedback current which is fed back to the superconducting detection loop for creating a magnetic flux that counter-acts the unknown magnetic field. The measurement of the magnetic field is achieved by counting up the number of the output pulses produced by the SQUID sensor until the unknown magnetic flux is canceled out by the feedback current. The foregoing single-chip SQUID magnetometer has an advantageous feature in that the feedback circuit, formed of a superconducting circuit, is provided within the cooling vessel commonly with the SQUID sensor. Thereby, one can minimize the penetration of heat into the cooling vessel via lines connecting the SQUID sensor in the cooling vessel to external circuits.
As an alternative approach to minimize the penetration of heat into the interior of the cooling vessel, one may provide a number of SQUID sensors in a common cooling vessel and apply a multiplexing process to the output pulses of the SQUID sensors. In fact, in the field of medical applications, there is a strong demand for a multi-channel SQUID magnetometer as mentioned previously.
For example, the Japanese Patent Application 2-336401 filed Nov. 30, 1990, and laid-open on Jul. 24, 1992, as Japanease Laid-Open Patent Publication 4-204278, describes a multi-channel SQUID magnetometer wherein the output pulses of a number of SQUID sensors are multiplexed in a superconducting multiplexer such that the output pulses of the foregoing plural channels are outputted to the exterior of the cooling vessel via a single line. Such a construction of the magnetometer is effective for reducing the number of interconnections and hence the penetration of heat into the interior of the cooling vessel.
FIG. 1 shows the principle of operation of a typical SQUID sensor used in the foregoing single-chip SQUID magnetometer, wherein the SQUID sensor has a generally asymmetric threshold characteristic or flux verses critical bias current characteristic as shown in FIG. 1.
Referring to FIG. 1, the vertical axis represents the bias current I.sub.c and the horizontal axis represents the unknown magnetic flux that interlinks with the superconducting detection loop of the magnetometer, wherein the rectangular regions surrounded by a continuous line and designated as MODE1, MODE0, MODE+1, . . . , represent the region wherein the Josephson junctions that form the SQUID sensor are held in the zero-voltage state. The different modes in FIG. 7 represent different states characterized by a different number of flux quanta stored in the SQUID sensor, and in each mode, the Josephson junctions experience a transition to a finite voltage state when the operational point has escaped from the rectangular region by crossing the characteristic line.
In operation, the bias current I.sub.c is set to have a magnitude substantially equal to a critical level above which the SQUID sensor causes a transition to the finite voltage state in the absence of the external magnetic flux. Thus, the bias current I.sub.c is set to have a magnitude such that the bias current I.sub.c swings between a level +i.sub.TH and a level -i.sub.TH, wherein +i.sub.TH and -i.sub.TH respectively represent the positive and negative thresholds of the transition of the SQUID sensor to the finite voltage state. Thus, as long as there is no magnetic flux .PHI..sub.x interlinking with the superconducting detection loop to which the SQUID sensor is coupled, the transition of the SQUID sensor does not occur. On the other hand, when there exists a positive magnetic flux +.PHI..sub.x as indicated in FIG. 1, the SQUID experiences a transition to the finite voltage state in response to the positive peak of the bias current I.sub.c and a positive output pulse is produced in response to such a transition. When there exists a negative magnetic flux -.PHI..sub.x as indicated also in FIG. 1, the SQUID experiences a transition to the finite voltage state in response the negative peak of the bias current I.sub.c and a negative output pulse is produced in response to such a transition.
In such a SQUID sensor, therefore, it will be noted that the polarity of the output pulse represents the direction of the magnetic flux that interlinks with the superconducting detection loop of the SQUID sensor. The SQUID sensor continues to produce the output pulses as long as there exists the magnetic flux .PHI..sub.x interlinking with the superconducting detection loop, and the magnitude of the flux .PHI..sub.x is determined by counting up the number of the output pulses while canceling out the flux .PHI..sub.x in response to each output voltage pulse as mentioned previously. When the magnetic flux .PHI..sub.x is completely canceled out, the SQUID sensor stops outputting the voltage pulses.
FIGS. 2(A)-2(E) show the operation of a multi-channel SQUID magnetometer wherein a plurality of SQUID sensors each having a characteristic of FIG. 1 are provided in a cooling vessel in correspondence to a plurality of channels, wherein FIG. 2(A) shows the waveform of the A.C. bias current I.sub.c used for driving the SQUID sensors, while FIGS. 2(B)-2(D) show the waveform of the output voltage pulses produced by the SQUID sensors in response to the transition to the finite voltage state. As will be noted, the SQUID sensors produce positive or negative output pulses in response to the direction or polarity of the magnetic flux .PHI..sub.x and the output pulses of FIGS. 2(B)-2(D) are multiplexed into a single output signal shown in FIG. 2(E).
When constructing such a multi-channel magnetometer, it is necessary to set the magnitude of the bias current I.sub.c to be equal to the threshold level i.sub.TH as mentioned previously for each of the SQUID sensors, wherein such an adjustment of the bias current I.sub.c has to be achieved with a precision of 1% or less with respect to the threshold level i.sub.TH. Because of such a small tolerance allowed in the magnitude of the bias current I.sub.c, one has to adjust the magnitude of the bias current with respect to individual SQUID sensors in the magnetometer, in view of the fact that the SQUID sensors of different channels may have the threshold levels that change device by device. However, it will be noted that such an adjustment of the bias current with respect to individual SQUID sensors makes the construction of the magnetometer extremely complex, and the construction of such a multi-channel SQUID sensor has been extremely difficult.